This invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium having a high degree of surface smoothness and orientation of magnetic particles (high squareness ratio), good running properties, and great durability.
A magnetic recording medium, such as audio cassette tape used to record music, required to have flat frequency characteristics and high fidelity in reproduction of the original sound. In addition, the increased use of auto stereophonic sound systems and radio/cassette tape recorders has made it more necessary that magnetic recording media have good running (referring to transport through a recording/reproduction apparatus) properties and great durability.
Very high-density recording is often performed on video cassette tape by shortening the recording wavelength or reducing the track width, and this requires magnetic tape that provides high output and faithful reproduction of the original picture (i.e., a high S/N ratio). A total tape thickness of less than 20 .mu.m being not uncommon, the widespread use of portable video tape recording (VTR) devices has made it mandatory to develop VTR tape which is capable of running unaffected for a period much longer than the conventional tape can achieve.
Thus, there is much demand for both audio and video tapes having a higher degree of surface smoothness and orientation of magnetic particles, better running properties and greater durability.
To meet this demand, various binder compositions have been proposed, but none of those proposed has been found to satisfy all of the requirements specified above. To provide good running properties and durability, many binder compositions include a thermoplastic polyurethane which is primarily a polyester polyurethane prepared by reacting poly(1,4-butylene adipate) with tolylene diisocyanate. But it is often difficult to provide a magnetic recording medium having satisfactory surface smoothness, orientation of magnetic particles, running properties, and durability from a binder composition containing a polyester polyurethane derived from poly(1,4-butylene adipate). A binder composition containing a polyester polyurethane prepared by reacting polyethylene adipate and 4,4'-diphenylmethane diisocyanate is described in U.S. Pat. No. 4,135,016, but does not satisfy all of the above requirements either.